Apparatus for etching of printing surfaces



Jan. 24, 1967 J. F. CROSFIELD 3,300,362

APPARATUS FOR ETGHING OF PRINTING SURFACES Filed Feb. 11 1965 2 Sheets-Sheet 1 lnvehtor 122m [TV 245 tlorne y 5 United States Patent 3,300,362 APPARATUS FOR ETCHING OF PRINTING SURFACES John Fothergill Crosfield, London, England, assignor to Crosfield Electronics Limited, London, England, a British company Filed Feb. 11, 1963, Ser. No. 257,402 Claims priority, application Great Britain, Feb. 15, 1962, 5,864/62 5 Claims. (Cl. 156-345) This invention relates to the etching of printing surfaces.

In a known process, printing surfaces are etched by a process involving exposing gelatinous tissue through a photographic positive of the image which is to be formed on the printing surface to harden the parts of the tissue exposed to light, in a graded manner, placing the exposed tissues over the printing surface, which is usually on a rotatable cylinder, developingthe tissue to wash away the soft parts, and then selectively etching away the underlying printing surface where the tissue has been hardened to varying extents by exposure, by rotation of the cylinder in an etching bath.

In abath of ferric chloride, the rate ofetching increases with'the dilution as the distilled water-enables the etching medium to penetrate the gelatinous tissue. The etching of the printing surface causes an apparentdarkening, i.e. reduction of light reflected back from the surface of the cylinder. It has been customary for the etching operation to be carried out by a skilled operator who first mixes a number of jugs of ferric chloride of different densities. He then judges the state of the etching by observing the degree of darkening of the gelatinous tissue (known as carbon tissue) and alters the density of the etching bath as different parts of the image become visible. He also judges the time at which dilution must stop, as otherwise the ferric chloride would eventually penetrate and etch areas which should remain untouched. The quality of the etched cylinder thus depends to a large extent on subjective opinion so that, for example, if two different operators were to etch cylinders with the same image, the two cylinders would produce different results on the printing press.

The production of consistent results in the etched cylinders is of particular importance in multicolour printing.

If the three or four colour-printing cylinders which are mounted on the press have not been etched in a uniform manner, a considerable amount of adjustment of the ink extender and solvent at each ink tank by trial and error may be necessary to produce the desired printing results. These adjustments may themselves cause further troubles during the printing process, for example in the drying system.

According to the present invention, the tissue is also exposed to an etching control patch and either the control patch has a characteristic pattern of exposure or a locating means which has an angular extent corresponding to that of the control patch rotates in synchronism with the cylinder; the method according to the invention further includes progressively reducing the strength of the etching medium throughout at least a portion of the etching process, photoelectrically scanning the circumferential strip of the cylinder and tissue which includes the exposure control patch, selecting from the resulting electrical signal portions thereof representing the passage of the exposure control patch, by means of the characteristic pattern of the exposure control patch or by sensing the locating means, and bringing into operation means for terminating the etching process when the selected sig nal reaches a value representing .a predetermined tonal value of the etched cylinder.

In this way the cylinders are produced in a more con- 3,309,362 Patented Jan. 24, 1967 tion of the signal which passes through to the control equipment is that produced by the passage of the bars past the photoelectric device. In another form, a patch of material which is not affected by the etching medium is applied to the cylinder and is sensed by a further photoelectric device and used to control a gating circuit through which the output of th first photoelectric device is applied to the control equipment. As before the latter signal passes through to the control equipment only for the portion of the period of rotation of the cylinder during which the test patch is passing the associated photoelectric device.

The invention will be better understood from the following description of one example, with reference to the accompanying drawings, in which:

FIGURE 1 shows the general arrangement of the cylinder and etching bath, and

FIGURES 2 and 3 are alternative forms of control circuit for operating the control valves of the apparatus.

To etch a printing cylinder with the apparatus shown in the drawings a piece of gelatinous tissue must first be placed in contact with a positive image of the picture in a flat frame and this is then exposed with a bright light source. Where the light penetrates the positive it hardens the tissue to an extent proportional to the amount of light falling on that area of the tissue. Thus a full tone value on the positive will prevent light hardening the tissue in that area, whilst a very light tone will allow sufficient light through to harden the tissue considerably. With the image there is exposed on to the tissue a control patch consisting of between 50 and bars. These bars must be present in a tone value equal to or very near to the lightest tone required to etch. The gelatinous tissue (also known as carbon tissue) is then wrapped around a copper cylinder in such a manner that the bars extend parallel to the axis of the cylinder. The unhardened areas of the tissue are then washed away in a developing process and the cylinder is then ready for etching. In FIGURE 1 the cylinder with the carbon tissue attached is shown by the reference numeral 1. The control patch is shown by 2. The lower half of the cylinder projects into the bath 3 which, in use, contains ferric chloride diluted with distilled water during the etching operation. The pipe 28 leading from a sump 29 through a pump 30 to the tank 3 and the return pipe 31 leading back to the sump are also shown in FIGURE 1, together with the inlet pipe 17 for the distilled water. The pipe 17 includes a valve 18 which is operated, through a delay device 19, by the start button 20.

As the cylinder rotates a narrow circumferential strip which includes the control patch is scanned by a scanning head including a lamp 4, the light from which passes through a lens 5 before striking the cylinder at a point on the said circumferential strip. The light reflected from this strip passes through a lens 6 before reaching the photocell 7, the output of which is applied to an amplifier and control circuit 8, the control circuit including gating means for selecting from the photocell output only the signal produced in the interval during which the control patch is passing the scanning head. When the signal from the scanning head reaches a predetermined a value, the output of the circuit 8 operates relay 9 which stops the dilution by closing valve 18 in the distilled water pipe 17. Also, through delayed relay 10 it opens a valve 11 to open the drain 12 and thereby empty the sump and bath 3; in addition, after a delay determined by the delay unit 32, a valve 15 is actuated to connect a spray pipe 13 to the water supply.

The circuits for the amplifier and control unit are shown in greater detail in FIGURE 2. In FIGURE 2, the output of the scanning head 7 is applied to a variable gain amplifier 21, the output of which is in turn applied to an amplitude-limiting circuit 22. The signal from the limiting circuit passes to a frequency filter 23 which is so tuned that it will pass signals at a frequency corresponding to that produced in the photocell output when the bars of the control patch 2 pass by the scanning head at an angular velocity corresponding to the standard rotational speed of the cylinder 1. The output signal from the tuned filter therefore contains a signal corresponding to only that portion of the cylinder periphery which contains the control patch. This signal is rectified in the rectifier 24 and the resultant signal is applied through a switch 25 to the relay 9 and the delayed relay 1t) described above. The switch 25 is an interlock switch which closes when the dilution cycle starts.

Operation of the push button 26 of FIGURE 2 causes a graded surface to slide under the scanning head and the gain of the amplifier 21 can then be set to produce a predetermined reading on the meter 27.

The operation of the apparatus shown in FIGURES 1 and 2 will now be described in more detail.

Once the carbon tissue has been wrapped around the cylinder 1 and the cylinder has been positioned above the etching tray with the scanning head in register with the cylindrical strip containing the control bars, the operator presses the start button which results in the rotation of the cylinder, the circulation of the ferric chloride through the tray and the operation of an air system which blows air through holes in the pipe 33 to bubble the ferric chloride into contact with the cylinder. After a further delay determined by the circuit 19, the valve 18 in the distilled water inlet pipe is opened after which the etching medium is continuously diluted. In the initial period, during which the etching medium is of high density, it can penetrate only the thinner parts of the gelatine which correspond to the heavier tones of the image to be reproduced. Once continuous dilution commences, the distilled water enables the etching medium to penetrate thicker layers of gelatine so that the tonal range of the etching progressively increases. At some point during the etching process, the bars of the control patch, which are represented on the gelatine as very light tones, begin to be visible on the surface of the cylinder, resulting in the appearance in the photoelectric cell output of a signal at the filter frequency. This operates the relay 9 which closes the valve 18, permitting etching to continue at a fixed low density until after a given interval determined by the delayed relay 10, the ferric chloride circulation stops, the drain valve llll is actuated to empty the tray 3 and the relay 10 actuates the valve 15 which connects the spray valve 13 to the source of water. This rinses the cylinder and tray.

The light bars on the cylinder may be rubbed out with charcoal once the etching has been completed.

As an alternative to using a filter circuit to detect a signal corresponding to the bars of the cylinder, a solid control spot 38 may be exposed on to the gelatine and a white patch 39 unaffected by the etching medium may be placed axially in line with the control spot. This is illustrated in FIGURE 3. Separate photoelectric cells are then used to scan the white patch and the control spot. The output of the photoelectric cell 40, which scans the control spot, is applied to a gate circuit 41. The output of the photoelectric cell 42 which scans the peripheral strip containing the white patch 39 is applied to a Schmitt trigger circuit 43 to derive a pulse of rectangular waveform for application to the gating circuit 41. This signal opens the gating circuit for the period during which the white patch 39 is passing the photoelectric cell 42 and thus allows through the gating circuit a signal from the photoelectric cell 40 which corresponds to the passage of the control spot 38 past the cell 40. The gating signal is applied to a variable gain amplifier 44 and thence to a peak measuring circuit 45 to derive a signal corresponding to the maximum amplitude of the photocell output during the passage of the control spot. The impedance transformer 46 matches the high output impedance of the peak measuring circuit to the low input impedance of the Zener diodes 47, 48, 49 and 50, the last three Zener diodes being associated with indicator lamps 51, 52 and 53. These diodes and lamps facilitate the correct adjustment of the amplifier gain and also serve to indicate the progress of the etching operation. Initially, the cylinder will appear to be light brown in colour and the photocell will have a relatively high output. The diodes are biased to conduct at different voltages. For example, the gain of the amplifier may be set so that when the gelatinous tissue is first placed on the cylinder and the whole of the circumference is light brown in colour, only the lamps 51 and 52 are illuminated. As the control spot begins to show as a dark brown spot on the cylinder, the voltage at the output of the impedance transformer will decrease a little and the lamp 52 will be extinguished. Further reductions in the signal as the control spot is etched will cause the lamp 51 to be extinguished, and finally the signal from the impedance transformer passes through the Zener diode 47 to operate the relays 9 and 10.

The use of the method and apparatus according to the invention produces greater consistency because it prevents differences in judgment as to when to stop the etch between diiferent operators or even between the same operator at different times.

When the circuit of FIGURE 3 is used, the white patch 39 may be replaced by a piece of magnetic tape at the side of the cylinder, the pick-up head being encased in plastic.

The above description has assumed that linear dilution of the etching bath takes place in a continuous manner during the principal portion of the etching cycle. This is not essential and if desired the dilution can take place in a non-linear manner (the valve 18 being controlled by a cam during the dilution stage) or distilled water may be added in small doses at regular intervals.

I claim:

1. Apparatus for etching a printing cylinder covered with a light-sensitive acid-resist tissue which has been exposed to the image to be reproduced and to a control patch having a characteristic pattern, comprising: an etching bath; means for supporting the cylinder above said bath and driving means for rotating the cylinder so that during etching an advancing longitudinal portion of the cylinder is contacted by said bath; a photoelectric scanning head adjacent the periphery of that portion of the cylinder which carries the part of the tissue exposed to the control patch; a filter circuit connected to said scanning head to receive the signal therefrom and passing only that portion of said signal having a waveform corresponding to the characteristic pattern; and means responsive to the filtered signal to control the termination of the etching process when said filtered signal reaches a level representing a predetermined tonal value.

2. Apparatus according to claim 1, for use with a printing cylinder covered with a tissue exposed to a control patch in the form of a number of spaced bars parallel to the axis of rotation of the cylinder, in which said filter circuit is a resonant filter tuned to a frequency determined by the spacing of said bars and the speed of rotation of the cylinder.

3. Appara us for etching a printing cylinder covered with a light-sensitive acid-resist tissue which has been exposed to the image to be reproduced comprising: an etching bath; means for circulating an etching medium through said bath; means supporting said cylinder above said bath and driving means rotating said cylinder so that during etching an advancing longitudinal portion of said cylinder is contacted by said etching medium; means for progressively adding a diluent to said etching medium in said bath; locating means rotating with said cylinder; a photoelectric scanning head adjacent the periphery of that portion of the cylinder which carries the part of the tissue exposed to the control patch; sensing means adjacent the path of said locating means for generating a timing signal corresponding to the passage of said locating means; a gating circuit controlled by said timing signal to pass only that portion of the signal from said scanning head which represents the passage of said control patch; and means responsive to the gated signal to control the termination of the etching process when said gated signal reaches a level representing a predetermined tonal value.

4. Apparatus according to claim 3, in which said loeating means is a patch of distinctive colour resistant to said etching medium applied to said cylinder and in which said sensing means is a further photoelectric scanning head.

5. Apparatus according to claim 3, in which said locating means is a patch of magnetisable material and in which said sensing means is an electromagnetic pickup.

References Cited by the Examiner J. TRAVIS BROWN, Acting Primary Examiner.

NORMAN G. TORCHIN, Examiner.

A. D. RICCI, Assistant Examiner. 

1. APPARATUS FOR ETCHING A PRINTING CYLINDER COVERED WITH A LIGHT-SENSITIVE ACID-RESIST TISSUE WHICH HAS BEEN EXPOSED TO THE IMAGE TO BE REPRODUCED AND TO A CONTROL PATCH HAVING A CHARACTERISTIC PATTERN, COMPRISING: AN ETCHING BATH; MEANS FOR SUPPORTNG THE CYLINDER ABOVE SAID BATH AND DRIVING MEANS FOR ROTATING THE CYLINDER SO THAT DURING ETCHIN AN ADVANCING LONGITUDINAL PORTION OF THE CYLINDER IS CONTACTED BY SAID BATH; A PHOTOELECTRIC SCANNING HEAD ADJACENT THE PERIPHERY OF THAT PORTION OF THE CYLINDER WHICH CARRIES THE PART OF THE TISSUE EXPOSED TO THE CONTROL PATCH; A FILTER CIRCUIT CONNECTED TO SAID SCANNING HEAD TO RECEIVE THE SIGNAL THEREFROM AND PASSING ONLY THAT PORTION OF SAID SIGNAL HAVING A WAVEFOAM CORRESPONDING TO THE CHARACTERISTIC PATTERN; AND MEANS RESPONSIVE TO THE FILTERED SIGNAL TO CONTROL THE TERMINATION OF THE ETCHING PROCESS WHEN SAID FILTERED SIGNAL REACHES A LEVEL REPRESENTING A PREDETERMINED TONAL VALUE. 